Low-temperature lattice instability in SnTe

The temperature dependences of the unit cell parameter a(T) of tin tellurid e with different extents of deviation from stoichiometry are obtained durin g heating from 80 to 290 K. The a(T) dependence for a sample with the stoic hiometric composition (50 at.% Te) displays an anomaly in the temperature r ange 90-100 K, which is attributed to the well-known ferroelectric phase tr ansition (PT). Well-pronounced jumps in the unit cell parameter (Delta a/a approximate to 0.015) observed for 50.4 at.% Te in the intervals 135- 150 K and 200-215 K correspond to a negative thermal expansion coefficient. Upon a further increase in the deviation from stoichiometry (50.8 at.%Te) these effects become less pronounced. The instability in the crystal lattice in certain temperature intervals is attributed to phase transitions in the sub system of intrinsic defects (nonstoichiometric vacancies! associated with t heir redistribution over the cation sublattice upon a change in the tempera ture and composition. The role of relaxation phenomena in the rearrangement of crystal defect subsystem is determined. (C) 1999 American Institute of Physics. [S1063-777X(99)00904-4].